Anticipating a gruesome or traumatic event makes it more vivid and deeply imprinted in the memory, a study says.
Volunteers had their brains scanned
Researchers found if people were aware something was going to happen, a key memory-forming part of the brain fired.
The University of Wisconsin-Madison said the findings may have implications in the treatment of conditions such as post-traumatic stress disorder.
The study, based on 36 people, is published in the Proceedings of the National Academy of Sciences journal.
Researchers showed the volunteers symbols before exposing them to a series of gruesome images, such as pictures of mutilated bodies, while linked up to MRI scanners.
The symbols were either neutral or signified that they were about to be shown something gruesome.
Half an hour after being shown the images, the volunteers were quizzed about how well they remember them.
Two weeks later, the volunteers were asked again.
On both occasions, those who had previously been given an indication that gruesome pictures were going to be shown were more likely to remember them.
After studying the MRI scans, researchers found two parts of the brain - the amygdala and hippocampus - were activated during the anticipation stage.
Scientists think the amygdala is associated with the formation of emotional memories, while the hippocampus helps the brain form long-term recollections.
Lead author Jack Nitschke said: "Our study illustrates how the power of expectancy can extend to memory formation as well.
"Just the expectation of seeing something bad can enhance the memory of it after it happens.
"In the future, we could look for ways to dampen that arousal response in patients so that they do not evoke negative memories so easily."
But psychologists said it could work both ways.
Dr Roderick Orner, of the British Psychological Society, said: "This is an understandable reaction and for some scenarios it is quite plausible.
"But we have also noticed that people can become conditioned to bad things happening.
"For example, if you live in an area where disasters are likely to happen, perhaps a war zone, and that disaster does happen those people may not be as traumatised as people affected in places where disasters don't happen."